The present invention is directed to plastic container closures for beverage, food, juice, pharmaceutical and like applications, and more particularly to an improved process for providing closures with sealing liners having transmission resistance to gases, water vapor and/or flavorants (flavor scalping).
Reference is made to concurrently filed application Ser. No. 08/998,072 filed Dec. 24, 1997 entitled xe2x80x9cPlastic Closure with Compression Molded Sealing/Barrier Linerxe2x80x9d and assigned to the assignee hereof.
It has heretofore been proposed to provide a plastic closure for a container that comprises a plastic cap with an interior liner for sealing engagement with the sealing surface of the container. For example, U.S. Pat. No. 4,984,703 discloses a plastic closure that comprises a cap having a base with a peripheral skirt and threads for securing the cap to a container, and a sealing liner compression molded in situ on the interior of the cap base. The sealing liner comprises a blend of ethylene vinyl acetate (EVA) and a thermoplastic elastomeric material such as olefin or styrene-butadiene-styrene. U.S. Pat. No. 5,451,360 discloses a method and apparatus for compression molding the liners in situ within the caps.
It has also heretofore been proposed to form a barrier liner for a plastic closure in an injection or extrusion molding operation from a blend of a thermoplastic olefin homopolymer or copolymer such as ethyl vinyl acetate (EVA) or thermoplastic elastomer, and a barrier polymer to oxygen and carbon dioxide transmission such as ethyl vinyl alcohol (EVOH). The blend also typically includes a compatiblizer material to promote adhesion between the EVOH particles and the EVA carrier material. In general, particles of the EVOH barrier material force gases that permeate the EVA carrier either to follow a tortuous path around the EVOH particles or permeate through the EVOH polymer, thus slowing the rate of gas transmission. The EVOH particles are generally spheroidally dispersed in the EVA carrier since these materials are not miscible. As a general proposition, the percentage reduction in gas transmission corresponds to the percentage content of EVOH in the blend. The results of this technology to date have not provided high barrier properties at acceptable cost and sealing performance.
It is therefore a general object of the present invention to provide a liner for a plastic closure that combines the functions of a seal for engagement with the container sealing surface and an improved barrier against gas transmission, flavor absorption (flavor scalping) and/or water vapor permeation. Another and more specific object of the present invention is to provide a liner of the described character that is of readily moldable and inexpensive composition. Yet another object of the invention is to provide a liner that satisfies the foregoing objectives and is of clear or translucent construction to permit reading through the liner of printing on the closure. A further object of the present invention is to provide a method of fabricating such a liner, and a plastic closure embodying such a liner.
In accordance with one aspect of the present invention, there is provided a plastic closure that comprises a cap having a base with a peripheral skirt defining a cap interior and means on the skirt for securing the closure to a container. A liner is secured to the interior of the cap, preferably by compression molding in situ. The liner consists essentially of a blend of a matrix polymer, a barrier material and a compatibilizer. The barrier material is in the form of a discontinuous distributed phase of particles in a continuous matrix phase. It has been found that the heat and pressure of compression molding forms the barrier polymer particles into thin overlapping substantially two-dimensional platelets that are oriented within the matrix polymer essentially parallel to the plane of the liner. Thus, gas that permeates the liner must follow a more tortuous path around the platelets, which greatly increases barrier efficiency.
The xe2x80x9cmatrix polymerxe2x80x9d is a thermoplastic elastomer, a soft olefin polymer, or a combination thereof. A thermoplastic elastomer is a synthetic polymer having the processability of a thermoplastic material and the functional performance and properties of a conventional theremoset rubber. There are six generic classes of thermoplastic elastomer commercially available, including styrenic block copolymers (SBC), polyolefin blends (TPO), elastomeric alloys, thermoplastic polyurethanes (TPU), thermoplastic copolyesters and thermoplastic polyamides. Thermoplastic elastomers are described beginning at page 64 in Modern Plastics Encyclopedia Handbook, published by McGraw-Hill, 1994, the disclosure of which is incorporated by reference. Examples of thermoplastic elastomers are styrene block copolymers as manufactured by Shell Chemical under the trademark KRATON. These synthetic polymers consist of three discrete blocks of the linear or A-B-A type: styrene-butadiene-styrene, styrene-isoprene-styrene, and styrene-ethylene/butylene-styrene-styrene. An elastomeric alloy is ethylene-propylene-diene terpolymer (EPDM). Another elastomeric alloy consists of compounds of EPDM/PP and butyl rubber/PP as manufactured by Advanced Elastomer Systems under the tradenames SANTOPRENE and TREFSIN and disclosed in U.S. Pat. Nos. 4,130,535, 4,311,628, 4,130,534 and 4,607,074. In general, thermoplastic elastomers are characterized by a Shore A hardness of 45 to 95 and a flexural modulus of 30,000 to 100,000 psi.
Soft olefin polymers are thermoplastic olefins, homopolymers and copolymers which are flexible, elastic with a Shore A hardness of less than about 100. Typical soft olefin polymers are: metallocene made polyethylene, ethylene-propylene rubbers, ethylene copolymers and blends thereof, ethylene copolymers such as ethylene vinyl acetate, ethylene methyl acrylate copolymers and ionomers, and combinations thereof. Examples of soft olefin polymers are alpha olefin substituted polyethylenes manufactured using single site catalyst technology (these materials are known in the art as metallocene made polyethylenes); ethylene vinyl acetate (EVA) such as manufactured by DuPont under the trademark ELVAX; polypropylene made with single site catalyst technology known in the art as metallocene made polypropylenes; syndiotactic polypropylenes as marketed by Fina Oil and Chemical; ethylene/propylene copolymers and styrene-ethylene interpolymers as marketed by Dow Chemical; and ionomers such as DuPont""s SURLYN product line.
The matrix polymer is typically compounded with anti-oxidants, lubricants and other stabilizing materials, as known in the art.
A xe2x80x9ccompatibilizerxe2x80x9d is a thermoplastic that ties two other thermoplastics together by a reactive (covalent or dipolexe2x80x94dipole) bond or a non-reactive (chain entanglement) means. Examples include maleic anhydride grafted polymers or ethylene vinyl acetate grafted polymers such as Quantum Chemical""s PLEXAR (trademark), Mitsui Petrochemical""s ADMER (trademark) and DuPont""s BYNEL (trademark) product lines, ethylene methyl acrylate, and ionomers.
A xe2x80x9cbarrier materialxe2x80x9d is a thermoplastic material that has a low gas and/or water vapor transmission rate, and presents a high barrier to odorants and essential oils. The following materials have gas transmission rates lower than EVA, which is an industry standard liner material: EVOH (ethylene vinyl alcohol) such as Nippon Goshei""s SOARNOL product line and Evalca""s EVAL (trademark) product line, nylons such as Dupont""s SELAR (trademark) PA, EMS""s G21 and Mitsubishi Gas"" MXD6 product lines, British Petroleum""s BAREX (trademark) acrylonitrile product line, blends of EVOH and amorphous nylon, blends of EVOH and an ionomer such as SURLYN (DuPont), and cyclic olefin copolymers such as marketed by Ticona. Other suitable barrier materials and blends are disclosed in U.S. Pat. Nos. 4,977,004 and 5,064,716, disclosures of which are incorporated herein by reference.
It is currently preferred that the liner also include an additive for reducing the coefficient of friction between the liner and the sealing surface of the container. In the art these additives are called xe2x80x9clubricants.xe2x80x9d Typical additive lubricants include fatty acid amides, and fatty acid esters, microcrystalline waxes and polyethylene glycols. A preferred lubricant is a low molecular weight fatty acid amide material that blooms to the exposed surface of the polymer material upon cooling from the melt state, thereby reducing the coefficient of friction between the liner and the container sealing surface. Examples are: primary amides with the general chemical structure Rxe2x80x94COxe2x80x94NH2, where R is an alkyl group; secondary amides with the general chemical structure Rxe2x80x94COxe2x80x94NHxe2x80x94Rxe2x80x2; where R, Rxe2x80x2 are alkyl groups; secondary bis-amides with the general chemical structure Rxe2x80x94COxe2x80x94NH-A-NHxe2x80x94COxe2x80x94R, where R, Rxe2x80x2 are alkyl groups and A is an Y alkylene group; and blends of the above materials such as in U.S. Pat. No. 5,306,542. The lubricant preferably comprises about 0.5% to 2.0% of the total liner composition by weight, most preferably about 1% by weight. The lubricant is preferably compounded into thermoplastic elastomer material (along with any desired colorants) by the material manufacturer. The amount of lubricant and/or colorant is not included in the calculations of compositions in this application.
In the currently preferred blends of matrix polymer, barrier material and compatibilizer, the barrier material is in the range of about 20% and 60% by weight, the compatibilizer in the range of about 1% to 10% by weight, and the balance consists essentially of the matrix polymer (with lubricant). The relative component percentages will vary with applications, and will depend upon hardness and therefore sealability, the ability of the barrier material to form platelets in the matrix polymer, and the desired removal torque. As to hardness, it has been found that a liner hardness higher than about 94 to 95 Shore (A) is too hard for proper sealing with the container. When employing an elastomer that is relatively hard, such as EVA, the upper limit of the barrier material may be relatively low, such as 35% EVOH. However, when employing a matrix polymer of relatively low hardness, such as thermoplastic elastomer, the upper limit of the barrier material may be much higher, such as 50% to 60% EVOH.
It has also been found that the amount of barrier material that can be included in the blend also depends in part upon polarity of the matrix polymer. Miscibility of the barrier material in the matrix material decreases with decreasing polarity of the matrix material, which means that higher percentages of barrier material can be blended with matrix polymers of lower polarity while still maintaining a discontinuous barrier platelet phase. The amount of compatibilizer also affects the ability of the barrier material to form platelets. Above about 10% compatibilizer, the barrier material becomes to well mixed with the matrix polymer to form the platelets that characterize the invention. Below about 1%, the liner does not exhibit good mechanical properties and does not bond properly with the closure shell.
In accordance with another aspect of the present invention, there is therefore provided a method for making a liner for a plastic closure that comprises the steps of forming a pellet that consists essentially of a blend of a thermoplastic elastomer, a polymer having high barrier properties and a compatibilizer to promote adhesion between the elastomer and the barrier polymer and between the liner and the closure shell. The pellet is then compression molded, preferably within a closure shell, to form a liner disk bonded to the closure shell, and within which the heat and pressure from compression molding forms the barrier polymer into platelets that are oriented essentially parallel to the plane of the disk. The resulting liner exhibits greatly lowered gas transmission rate as compared with a liner formed of a similar blend in other than a compression molding operation.
In accordance with a third aspect of the invention, there is provided a sealing liner for a plastic closure that comprises a disk that consists essentially of a matrix polymer within which a dispersion of platelets of a barrier material to gas transmission are disposed essentially in planes parallel to the disk. The sealing liner preferably is compression molded in situ within a plastic closure from a compression mold charge or pellet in which the matrix polymer, the barrier material and a compatibilizer are blended such that the barrier material consists of discrete particles that are flattened into platelets during the compression molding operation.